This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In the presence of water aligned lipid membranes self-assemble in the vicinity of flat and smooth surfaces forming a smectic-C liquid crystal structure with water intercalated between the bilayers formed by the lipid molecules. By sandwiching hydrated lipids in between two substrates one can achieve several square-millimeter large defect-free monodomains of highly aligned lipid membranes. Using thin silicon-nitride windows as alignment substrate transmission SAXS experiments on such samples are possible. The small size and high intensity of the beam available at bealine 4-2 together with the large defect free domain size allows to probe the structure in the plane of the membrane. In this project we investigated the change in the nearest neighbor distance between membrane embedded peptides in dependence on the thickness of the lipid and the peptide concentration. We find that the average distance between the proteins increases with increasing thickness of the bilayer. This effect is attributed to the increased membrane mediated repulsive interaction between the peptides due to the increased hydrophobic mismatch. The data can be fitted using the product of the known formfactor of the peptide and a theoretical structure factor calculated assuming hard-core repulsion between the embedded peptides. The hard core repulsion was found to be an adequate model for the system with the shortest lipid chains. An increase in the lipid chain length producing a mismatch between the hydrophobic lengths of the peptide and the lipid causes the onset of longer ranged, membrane mediated forces between the peptides. In order to include these additional forces into the hard-core structure factor a perturbation extension of the current model is currently being pursued.